Substrate, metrology apparatus and associated methods for a lithographic process

ABSTRACT

A substrate having a plurality of features for use in measuring a parameter of a device manufacturing process and associated methods and apparatus. The measurement is by illumination of the features with measurement radiation from an optical apparatus and detecting a signal arising from interaction between the measurement radiation and the features. The plurality of features include first features distributed in a periodic fashion at a first pitch, and second features distributed in a periodic fashion at a second pitch, wherein the first pitch and second pitch are such that a combined pitch of the first and second features is constant irrespective of the presence of pitch walk in the plurality of features.

This application claims the benefit of priority of European patentapplication no. EP17188175, filed Aug. 28, 2017, which is incorporatedherein in its entirety by reference.

FIELD

The present disclosure relates to a substrate, a metrology apparatus andassociated methods for a lithographic process. In particular, theinvention may relate to, but need not be limited to, a substrate,metrology apparatus and associated methods for measuring pitch walk in alithographic process.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in device manufacturingprocesses, such as in the manufacture of integrated circuits (ICs). Inthat instance, a patterning device, which may alternatively be referredto as a mask or a reticle, may be used to generate a circuit pattern tobe formed on an individual layer of the IC. This pattern can betransferred onto a target portion (e.g., comprising part of, one, orseveral dies) on a substrate (e.g., a silicon wafer). Transfer of thepattern is typically via imaging onto a layer of radiation-sensitivematerial (resist) provided on the substrate. A single substrate mayinclude a network of adjacent target portions that are successivelypatterned. Lithographic apparatus may include a stepper and/or scanner.A stepper may be configured such that each target portion is irradiatedby exposing an entire pattern onto the target portion at one time. Ascanner may be configured such that each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor a device manufacturing process such as alithographic process, one or more parameters of the patterned substrate(and therefore of any aspect of the device manufacturing process thataffects the patterned substrate) may be measured. The one or moreparameters may include, for example, the overlay error betweensuccessive layers formed in or on the patterned substrate and/orcritical dimension (e.g. linewidth, or the like) of developedphotosensitive resist and/or etched product features. One or more of theparameters may include feature heights and/or feature pitches. Thesemeasurements may be performed on a product substrate and/or on adedicated metrology target. There are various techniques for makingmeasurements of the structures formed in lithographic processes,including the use of scanning electron microscopes and variousspecialized tools. A fast and non-invasive form of specializedinspection tool is a scatterometer in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered or reflected beam are measured. By comparing properties ofthe beam before and after it has been reflected, diffracted and/orscattered by the substrate, one or more properties of the substrate maybe determined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withone or more known substrate properties or data calculated in real timefrom a model of the scattering structure.

SUMMARY

Multiple patterning is a class of techniques used to increase featuredensity. In double patterning, for example, a lithographic process isenhanced to halve a minimum spacing between separate features. Inquadruple patterning, a lithographic process is enhanced to reduce theminimum spacing by a factor of four.

Spacer patterning, which may also be referred to as Spacer ProcessTechnology (SPT), is a multiple patterning technique in which layers areformed on sidewalls of pre-patterned (or mandrel) features. Thepre-patterned features are subsequently removed to leave two residualsidewall features for each pre-patterned feature. Where the widths ofthe pre-patterned features are exactly equal to the separations betweenthe pre-patterned features, features formed using the residual sidewallfeatures will be spaced apart from each other with a single, commonseparation distance. Errors in the spacer patterning process can,however, cause the separation between adjacent features to vary. In thecase of double patterning, the variation may comprise a separationdistance that alternates. An alternating separation distance may bereferred to as pitch walk. Pitch walk may occur for example when anerror in the lithographic process causes the width of the pre-patternedfeatures to be different to the separation between the pre-patternedfeatures. Pitch walk may also arise in other forms of multiplepatterning, for example in non-spacer techniques such asLitho-Etch-Litho-Etch (LELE).

Pitch walk may be measured in special target patterns in a substratewhere a bias is introduced in either the CD or the pitch. However, thebias introduced is often larger than tolerated by design rules/processwindows for a lithographic process. Alternatively, device structures maybe measured at multiple stages of the process by a step that providesmeasurement of process parameters. The process parameters may bemeasured at different stages of the lithographic process. The processparameter may be combined together to “estimate” the pitch walk at thefinal patterning step. However, many recipes may have to be made, eachof which may have an individual error that may to introduce a largeerror in the estimation. This method may be logistically difficult dueto the number of steps involved. For example, significant effort inmaking multiple metrology recipes (e.g. a SPT process such asself-aligned quadruple patterning (SAQP) might require 4-5 steps, forexample) may be required. Each recipe may have its own metrology bias.When all measurements are combined this bias can overshadow the accuracyrequired for pitch walk metrology.

Accurate measurement of one or more properties of device manufacturingprocesses, including measurement of pitch walk resulting from multiplepatterning process, can be difficult, time consuming, or both.

According to an example of the present disclosure there is provided asubstrate comprising a plurality of features for use in measuring aparameter of a device manufacturing process by illumination of thefeatures with measurement radiation from an optical apparatus anddetecting a signal arising from interaction between the measurementradiation and the features, wherein the plurality of features comprisefirst features distributed in a periodic fashion at a first pitch, andsecond features distributed in a periodic fashion at a second pitch, andwherein the first pitch and second pitch are such that a combined pitchof the first and second features is constant irrespective of thepresence of pitch walk in the plurality of features.

The use of first and second features having a constant combined pitchallows parameters S1 and S2, which can be used to define pitch walk, tobe independently detectable using specular reflected radiation. This inturn allows accurate determination of pitch walk.

Optionally, the first features have been fabricated using a spacerpatterning method and the combined pitch is a multiple of a width of amandrel used in the spacer patterning method.

Optionally, the first and second features are spatially coincident in xand y dimensions of the substrate.

Optionally, the first and second features are fabricated on separatelayers formed in or patterned on the substrate.

Optionally, the first and second features are fabricated on a singlelayer formed in or patterned on the substrate.

Optionally, one or more first features comprises a second feature, theheight of which has been at least partially reduced.

Optionally, the first features have a reduced height compared to thesecond features over at least part of a length and/or width of the firstfeatures in x and y dimensions of the substrate, wherein the height ofthe features is defined along a corresponding z-axis, normal to thesubstrate.

Optionally, a ratio of the height of the first features to the secondfeatures is 0.9 or less; 0.8 or less; 0.7 or less; 0.6 or less; or 0.5or less.

Optionally, the height of the first features to the second features is0.1 or more; 0.2 or more; or 0.3 or more.

Optionally, the first features have been formed by removal of at leastpart of one or more of the second features to reduce their height, or bydepositing at least one layer on one or more of the second features toincrease their height.

Optionally, the first and second structures form a plurality ofrepeating periodic unit cell structures, each unit cell structurecomprising at least one first feature and at least one second feature.

Optionally, the distribution of the at least one first and secondfeatures in the unit cell is such that the unit cell is symmetric.

Optionally, the unit cell has a width and/or length that is less than150 nm; less than 100 nm; less than 80 nm; less than 60 nm; or less than40 nm.

Optionally, the first and second features are arranged such that asignal comprising specular reflected radiation arising from interactionbetween the measurement radiation and the first and second featurescomprises 0^(th) diffraction order and is measureable using the opticalapparatus to determine the parameter of the device manufacturingprocess.

Optionally, the first and second features are arranged such that a pitchwalk error of the first and/or second features produces distinguishablepupil images in the pupil plane.

Optionally, the first and second features comprise lines fabricated inor patterned on the substrate by the device manufacturing process.

Optionally, the first pitch is greater than the second pitch and isoptionally a multiple of the second pitch.

Optionally, the second pitch is less than 100 nm; less than 80 nm; lessthan 60 nm; less than 40 nm; less than 20 nm; or less than 10 nm.

A substrate comprising a plurality of features for use in measuring aparameter of a device manufacturing process by illumination of thefeatures with measurement radiation from an optical apparatus anddetecting a signal arising from interaction between the measurementradiation and the plurality of features, wherein the plurality offeatures are distributed in a periodic fashion defining a common pitchof less than 100 nm between adjacent features, and wherein one or morefirst features of the plurality of features has an at least partiallyreduced height compared to one or more second features of the pluralityof features.

According to an example of the present disclosure there is provided ametrology apparatus for measuring a parameter of a device manufacturingprocess based on information acquired by an optical apparatus, theoptical apparatus configured to illuminate a plurality of features of asubstrate with measurement radiation and to detect a signal comprisingspecular reflected radiation arising from interaction between themeasurement radiation and the plurality of features by measuring thesignal using the optical apparatus, wherein the plurality of featurescomprise first features distributed in a periodic fashion at a firstpitch, and second features distributed in a periodic fashion at a secondpitch, and wherein the first pitch and second pitch are such that acombined pitch of the first and second features is constant irrespectiveof the presence of pitch walk in the plurality of features, themetrology apparatus comprising: a processor configured to: determine anexpected distribution of the specular reflected radiation, based on amodel; compare a measured distribution of the specular reflectedradiation with the determined expected distribution of the specularreflected radiation to determine an error therebetween; and if the erroris below a threshold, determine the parameter to be a parameterassociated with the model; or if the error is above a threshold, updatethe model.

Optionally, the measured distribution of the specular reflectedradiation is measured in the pupil plane.

Optionally, the specular reflected radiation comprises 0^(th) orderdiffracted radiation.

According to an example of the present disclosure there is provided amethod of measuring a parameter of a device manufacturing process basedon information acquired by an optical apparatus, the optical apparatusconfigured to illuminate a plurality of features of a substrate withmeasurement radiation and to detect a signal comprising specularreflected radiation arising from interaction between the measurementradiation and the plurality of features by measuring the signal usingthe optical apparatus, wherein the plurality of features comprise firstfeatures distributed in a periodic fashion at a first pitch, and secondfeatures distributed in a periodic fashion at a second pitch, andwherein the first pitch and second pitch are such that a combined pitchof the first and second features is constant irrespective of thepresence of pitch walk in the plurality of features, the methodcomprising: determining an expected distribution of the specularreflected radiation, based on a model; comparing a measured distributionof the specular reflected radiation with the determined expecteddistribution of the specular reflected radiation to determine an errortherebetween; and

if the error is below a threshold, determining the parameter to be aparameter associated with the model; or if the error is above athreshold, updating the model.

According to an example of the present disclosure there is provided acomputer program comprising instructions which, when executed on atleast one processor, cause the at least one processor to control anapparatus to carry out any method disclosed herein.

According to an example of the present disclosure there is provided acarrier containing the computer program above, wherein the carrier isone of an electronic signal, optical signal, radio signal, ornon-transitory computer readable storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present disclosure will now be described, by way ofexample only, with reference to the accompanying schematic drawings, inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a lithographic cell or cluster;

FIG. 3 depicts a scatterometer used in metrology;

FIGS. 4-9 schematically depict stages in an example double patterningprocess using spacer patterning;

FIG. 10 depicts in schematic side section a periodic target structureformed by double patterning, with zero pitch walk;

FIG. 11 depicts in schematic side section a periodic target structureformed by double patterning, with non-zero pitch walk;

FIGS. 12a and 12b respectively depict schematic elevated/plan views of asubstrate including a plurality of products and an expanded view of partof one of the products;

FIGS. 13 a, 13 b and 13 c respectively depict cross-section views ofexamples of substrates having different structural characteristicsaccording to an example of the present disclosure;

FIG. 14 depicts an elevated/plan view of another example of a substratehaving a different structural characteristic to the substratesillustrated by FIGS. 13 a, 13 b and 13 c;

FIGS. 15a and 15b respectively depict a cross-section view of asubstrate similar to the example of FIG. 13b without and with pitch-walkof the features;

FIGS. 16a and 16b depict a system for determining a parameter accordingto an example of the present disclosure;

FIG. 17 depicts a method for determining a parameter according to anexample of the present disclosure;

FIGS. 18 a, 18 b and 18 c respectively illustrate: a cross-section viewof a substrate W that is the same as FIG. 13a ; and two set-get plotsindicating an expected and simulated result for pitch-walk parametersusing the system or method of FIG. 16 a, FIG. 16b or FIG. 17; and

FIGS. 19 a, 19 b and 19 c respectively illustrate: a cross-section viewof a substrate W that is the same as FIG. 13b ; and two set-get plotsindicating an expected and simulated result for pitch-walk parametersusing the system or method of FIG. 16 a, FIG. 16b or FIG. 17.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an example of a lithographic apparatus LA.The lithographic apparatus LA includes an illumination system (e.g.illuminator) IL configured to condition a radiation beam B (which maycomprise UV, DUV, EUV radiation and/or any other wavelengths), a supportstructure (e.g., a mask table) MT constructed to support a patterningdevice (e.g., a mask) MA and connected to a first positioner PMconfigured to accurately position the patterning device in accordancewith certain parameters, a substrate table (e.g., a wafer table) WTconstructed to hold a substrate (e.g., a resist coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters, and a projectionsystem (e.g., a refractive projection lens system) PS configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g., comprising one or more dies) of thesubstrate W.

The illumination system IL may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic, or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The support structure supports the patterning device MA in a manner thatdepends on the orientation of the patterning device MA, the design ofthe lithographic apparatus LA, and other conditions, such as for examplewhether or not the patterning device MA is held in a vacuum environment.The support structure MT can use mechanical, vacuum, electrostatic orother clamping techniques to support the patterning device MA. Thesupport structure MT may include a frame, table, or the like, which maybe fixed or movable as required. The support structure MT may ensurethat the patterning device MA is at a desired position, for example withrespect to the projection system PS. Any use of the terms “reticle”herein may be considered synonymous with the more general term“patterning device”.

The term “patterning device” used herein may refer to any device thatcan be used to impart a radiation beam with a pattern in itscross-section such as to create a pattern in a target portion of thesubstrate. It should be noted that the pattern imparted to the radiationbeam may not exactly correspond to the desired pattern in the targetportion of the substrate, for example if the pattern includesphase-shifting features or so-called assist features. In an example, thepattern imparted to the radiation beam may correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples ofpatterning devices include reticles, masks, programmable mirror arrays,and programmable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam B in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein may refer to various types ofprojection system PS, including refractive, reflective, catadioptric,magnetic, electromagnetic and electrostatic optical systems, or anycombination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of an immersion liquid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system.”

In this example, the lithographic apparatus LA is of a transmissive type(e.g., employing a transmissive patterning device MA). Alternatively,the lithographic apparatus LA may be of a reflective type (e.g.,employing a programmable mirror array of a type as referred to above, oremploying a reflective mask).

The lithographic apparatus LA may be of a type having two (dual stage)or more substrate tables and, for example, two or more patterning devicetables. In such “multiple stage” machines the additional tables may beused in parallel, or preparatory steps may be carried out on one or moretables while one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate W may be covered by a liquid having arelatively high refractive index, e.g., water, so as to fill a spacebetween the projection system and the substrate. An immersion liquid mayalso be applied to other spaces in the lithographic apparatus, forexample between the patterning device MA and the projection system PS.Immersion techniques may be used for increasing the numerical apertureof projection systems.

Referring to FIG. 1, the illuminator IL receives a radiation beam B froma radiation source SO. The source and the lithographic apparatus LA maybe separate entities. In an example, the source may not be considered toform part of the lithographic apparatus LA such that the radiation beamB is passed from the source SO to the illuminator IL with the aid of abeam delivery system BD comprising, for example, suitable directingmirrors and/or a beam expander. In another example, the source SO may bean integral part of the lithographic apparatus LA. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (which are commonly referred to asa-outer and a-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device (e.g.,patterning device MA), which is held on the support structure (e.g.,mask table) MT, and is patterned by the patterning device MA. Havingtraversed the patterning device MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor, or the like), the substrate table WT canbe moved accurately, e.g., so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the patterning device MA with respectto the path of the radiation beam B, e.g., after mechanical retrievalfrom a mask library, or during a scan. In general, movement of thesupport structure MT may be realized with the aid of a long-strokemodule (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) thesupport structure MT may be connected to a short-stroke actuator only,or may be fixed. Patterning device MA and substrate W may be alignedusing patterning device alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these may be referred to as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the patterning device MA, the patterning device alignment marks maybe located between the dies.

The depicted lithographic apparatus LA may be used in at least one ofthe following modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-) magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell orcluster, which also includes apparatus to perform pre- and post-exposureprocesses on a substrate W. Conventionally these include one or morespin coaters SC to deposit resist layers, one or more developers DE todevelop exposed resist, one or more chill plates CH and/or one or morebake plates BK. A substrate handler, or robot, RO picks up substratesfrom input/output ports I/O1, I/O2, moves them between the differentprocess apparatuses and delivers then to the loading bay LB of thelithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU that is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it may be desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments, for example, can be made toexposures of subsequent substrates, especially if the inspection can bedone soon and fast enough that other substrates of the same batch arestill to be exposed. Also, already exposed substrates may be strippedand reworked to improve yield, or possibly be discarded, therebyavoiding performing exposures on substrates that are known to be faulty.In a case where only some target portions of a substrate are faulty,further exposures can be performed only on those target portions thatare deemed to be non-faulty.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine the properties of the substrates, forexample, how the properties of different substrates or different layersof the same substrate vary from layer to layer. The inspection apparatusmay be integrated into the lithographic apparatus LA, the lithocell LC,or any other apparatus, or may be a stand-alone device. A plurality ofinspection apparatus may be provided for a production line formanufacturing devices. For example, there may be at least one inspectionapparatus at one or more stages of the production line. To enable rapidmeasurements, it may be desirable for the inspection apparatus tomeasure properties in the exposed resist layer immediately after theexposure. However, the latent image in the resist may have a very lowcontrast, as in there may only be a very small difference in refractiveindex between the parts of the resist which have been exposed toradiation and those which have not. Some inspection apparatus may nothave sufficient sensitivity to make useful measurements of the latentimage. Measurements may be taken after the post-exposure bake step (PEB)that is customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image, at which point either the exposed or unexposed parts ofthe resist have been removed, or after a pattern transfer step such asetching. The latter possibility may limit the possibilities for reworkof faulty substrates but may still provide useful information. It willbe appreciated that the inspection apparatus may be used to performmeasurements during any appropriate stage of the lithographic process.It will also be appreciated that the inspection apparatus may beconfigured to perform measurements on at least one feature of thesubstrate, for example, a product feature, dedicated metrologytarget/feature, and/or the like.

FIG. 3 is a schematic diagram of an optical apparatus in the form of ascatterometer suitable for performing metrology in conjunction with thelithocell of FIG. 2. The apparatus may be used for measuring criticaldimensions of features formed by lithography, measuring overlay betweenlayers, measuring pitch walk, and/or the like. A product feature ordedicated metrology target may be formed on substrate W. The apparatusmay be a stand-alone device or incorporated in either the lithographicapparatus LA, e.g., at the measurement station, the lithographic cellLC, or at any appropriate location in a production line. An opticalaxis, which has several branches throughout the apparatus, isrepresented by a dotted line O. In this example, light emitted by source11 is directed onto substrate W via a beam splitter 15 by an opticalsystem comprising lenses 12, 14 and objective lens 16. These lenses arearranged in a double sequence of a 4F arrangement. A different lensarrangement can be used, provided that it still provides an image of thesource on the substrate, and simultaneously allows for access of anintermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can beselected by defining a spatial intensity distribution in a plane thatpresents the spatial spectrum of the substrate plane, here referred toas a (conjugate) pupil plane. This can be done by inserting an apertureplate 13 of suitable form between lenses 12 and 14, in a plane that is aback-projected image of the objective lens pupil plane. For example, asillustrated, aperture plate 13 can take different forms, two of whichare labeled 13N and 13S, allowing different illumination modes to beselected. The illumination system in the illustrated example forms anoff-axis illumination mode. In the first illumination mode, apertureplate 13N provides off-axis from a direction designated, for the sake ofdescription only, as ‘north’. In a second illumination mode, apertureplate 13S is used to provide similar illumination, but from an oppositedirection, labeled ‘south’. Other modes of illumination are possible byusing different apertures. The rest of the pupil plane may be dark asany unnecessary light outside the desired illumination mode mayinterfere with the desired measurement signals.

At least the 0th and potentially at least one of the −1 and +1 (andpotentially higher) orders diffracted by the target on substrate W maybe collected by objective lens 16 and directed back through beamsplitter 15. A second beam splitter 17 divides the diffracted beams intotwo measurement branches. In a first measurement branch, optical system18 forms a diffraction spectrum (pupil plane image) of the target onfirst sensor 19 (e.g., a CCD or CMOS sensor) using the 0th andpotentially first and higher order diffractive beams. Each diffractionorder may be incident on a different point on the sensor, so that imageprocessing may be capable of measuring, comparing and/or contrastingorders. The pupil plane image captured by sensor 19 may be useable forfocusing the metrology apparatus and/or normalizing intensitymeasurements of the first order beam. The pupil plane image may be usedfor any appropriate measurement purpose, for example, reconstruction offeatures, or the like. As explained herein, the pupil plane image may beused for measuring properties of specular reflected radiation (e.g.including at least a 0^(th) diffractive order) to determine at least oneparameter of a device manufacturing process.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate Won sensor 23 (e.g., a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the −1 or +1 firstorder beam. The image detected by sensor 23 is thus referred to as a‘dark-field’ image. Note that the term ‘image’ is used here in a broadsense. An image of the grating lines as such will not be formed, if onlyone of the −1 and +1 orders is present.

The images captured by sensors 19 and 23 are output to image processorand controller PU, the function of which will depend on the particulartype of measurements being performed.

Examples of scatterometers and techniques can be found in patentapplications US 2006/066855 A1, WO 2009/078708, WO 2009/106279, and US2011/0027704 A, the contents of all of which are incorporated byreference herein in their entirety.

In the following, methods of measuring a parameter of a devicemanufacturing process, particularly of a lithographic process, accordingto examples are described. The methods may be applicable to measuring aparameter of a lithographic process comprising multiple patterning, forexample double patterning (e.g. self-aligned double patterning (SADP),or the like) or quadruple patterning (e.g. self-aligned quadruplepatterning (SAQP), or the like). An example of a double patterningprocess using spacer patterning is described below with reference toFIGS. 4-9. At least one example of the present disclosure may be usedwhere other forms of multiple patterning are used (spacer or non-spacer)and even in processes which do not involve multiple patterning.

FIG. 4 depicts a substrate 34. A base layer 32 is formed on thesubstrate 34. A patterned layer comprising a plurality of pre-patternedfeatures or mandrels 30 (e.g. lines), forming a first pattern, is formedon the base layer 32.

In a subsequent step, as depicted in FIG. 5, a film layer 35 isdeposited onto the patterned layer.

In a subsequent step, as depicted in FIG. 6, etching is performed toremove material from the film layer 35 on the horizontal surfaces.Layers 36 are thereby formed on sidewalls of the pre-patterned features30. The layers 36 may be referred to as spacers. As can be seen, thedistance between the spacers 36 is dependent on the width of thepre-patterned features 30 and also the distance between adjacent ones ofthe preformed features. The pitch between adjacent spacers 36 istherefore a function of the width and/or the space between adjacentpre-patterned features 30.

In a subsequent step, as depicted in FIG. 7, the pre-patterned features30 are removed leaving the spacers 36 forming a pattern having twice thedensity of the original pattern of pre-patterned features 30 (becauseeach of the pre-patterned features 30 had two sidewalls and eachsidewall produces one of the spacers 36).

In a subsequent step, as depicted in FIG. 8, the spacers 36 are used asa mask to define selective etching of the base layer 32 to form a secondpattern of features 38.

In a subsequent step, as depicted in FIG. 9, the spacers 36 are removed,leaving the second pattern of features 38 formed by the remainingmaterial of the base layer. The second pattern (shown in FIG. 9)comprises twice as many features as the first pattern (shown in FIG. 4).

The process described above with reference to FIGS. 4-9 is sometimesreferred to as Self-Aligned Double Patterning (SADP). The process can berepeated based on the features 38 of the second pattern instead of thefirst pattern, thereby doubling the feature density a second time. Thistype of process is sometimes referred to as Self-Aligned QuadruplePatterning (SAQP). The process can in principle be repeated further toproduce further increases in feature density.

Referring to FIG. 9, the separation distances S1 are determined by thewidth of the pre-patterned features 30. The separation distances S2 aredetermined by the separation distances between adjacent pairs of thepre-patterned features 30. A difference between S1 and S2 will result inan effective overlay error between odd and even features 38. Theseparation distance between adjacent features for a case where S1-S2 isnon-zero will therefore alternate. An alternating separation distancemay be referred to as pitch walk. FIG. 10 depicts a portion of astructure with zero pitch walk (S1=S2). FIG. 11 depicts a portion of astructure with non-zero pitch walk (S1≠S2). It may be desirable tomonitor and control any difference between S1 and S2 (e.g. to ensure thedifference does not exceed a predetermined threshold). It is noted thatthe summation of S1 and S2 remains essentially constant even if there ispitch walk because of the method of fabrication of the features 38described above.

Techniques for measuring S1-S2 (and therefore pitch walk) may havevarious shortcomings.

Scanning electron microscopy (CD-SEM) may be used to measure S1-S2.Scanning electron microscopy is, however, relatively slow (typicallyrequiring several seconds to make the measurement). CD-SEM measures withhigh localization on the device, meaning that a large number ofinspection points may be required to inspect the aggregate targetperformance. Furthermore, it may be difficult to discriminate between S1and S2.

Scatterometric techniques provide improved speed but can have lowsensitivity, particularly for small values of S1-S2.

The structures defined by the features 38 (and the separation thereof)in FIGS. 10 and 11 are such that the specular reflected radiation doesnot yield any information, or at least does not provide sufficientinformation, to determine the value of S1 or S2 (e.g. to determine thelevel of pitch-walk, and/or another parameter) by analyzing informationobtained at the pupil plane of the optical apparatus (e.g. by using thefirst sensor 19 of the scatterometer in FIG. 3).

However, it will be appreciated that the structure (e.g. CD, or thelike), configuration, distribution and/or any other parameter associatedwith the features 38 in FIG. 11 may still be capable of being measuredin the pupil plane or another plane of the optical apparatus. Thus, inthe example of FIGS. 10 and 11, it may still be possible to determine atleast one parameter from a measurement of the signal in the pupil planeand/or any other plane of the optical apparatus.

An example of a method of measuring a parameter (such as pitch walk,and/or at least one other parameter) of a device manufacturing process(e.g. part of a lithographic process) based on information acquired bythe optical apparatus (for example, using the scatterometer of FIG. 3,or another appropriate optical apparatus) is described below. Thepresent disclosure recognizes that by providing a substrate W includingfeatures with certain structural characteristics, it may be possible tomeasure the parameter (such as pitch walk, and/or other parameters)using specular reflected radiation arising from interaction betweenmeasurement radiation and the substrate W. The specular reflectedradiation may include 0th order radiation after interaction with thefeatures. If present, 1st and/or higher diffractive orders may affect atleast one property of the specular reflected radiation, for example, viainteraction of an evanescent wave propagating through at least a part ofthe substrate W and the specular reflected radiation. The specularreflected radiation may comprise the 0th diffractive order of the signalarising from interaction between the measurement radiation and thefeatures.

The method could use product features on a substrate W to measure theparameter. For example, a product formed on the substrate W may includefeatures having certain characteristics corresponding to at least one ofthe examples described herein that enables the measurement of theparameter using only or at least the specular reflected radiation. In anexample, the method may be capable of measuring the parameter usinginformation present in the higher diffractive orders (e.g. 1st and/orhigher orders which may be produced by the substrate W). It will beappreciated that the method may be capable of measuring or determiningthe parameter using at least one diffractive order (e.g. at least oneselected from: the 0th, ±1st, or higher diffractive orders) in the pupilplane. The information yielded by the different diffractive orders maydepend on the certain characteristics of the features. For example,different CD values, spacing values (e.g. S1 & S2), pitch values, andthe like, may influence which diffractive order(s) contain relevantinformation that can be used to determine the parameter or parameters ofinterest.

The features having the certain characteristics may define at least onetarget. The term “target” used herein may refer to any structure that isused, or capable of being used, in a measurement process. A target maycomprise a dedicated metrology target or a target may form part of astructure that is partially or completely provided for other purposes. Atarget may for example be formed from product features.

FIGS. 12a and 12b respectively depict schematic views of a substrate Wincluding a plurality of products 40 and an expanded view of part 42 ofone of the products 40. The part 42 includes a plurality of features,which in this example are in the form of spaced-apart lines 44 extendingalong a Y-axis of the substrate W and distributed in a periodic fashionalong the X-axis. It will be appreciated that the distribution, size andconfiguration of products 40 on the substrate W and indeed the substrateW itself is purely schematic and not-to-scale. Further, thedistribution, size and configuration of the lines 44 is purely schematicand not-to-scale.

FIGS. 13 a, 13 and 13 c respectively depict different examples offeatures on the substrate W having at least one structuralcharacteristic that provides a part 42 comprising a plurality offeatures (e.g. the lines 44 of FIG. 12 b, or the like) for use inmeasuring a parameter of a device manufacturing process. These examplesdepict cross-sectional views of the lines 44 (e.g. as depicted by thesection A-A of FIG. 12b ) such that the lines 44 are viewed in adirection along the Y-axis. The examples of FIGS. 13a-c illustrate lines44 with different configurations (or patterns) of lines 44 havingheights h (e.g. defined along the Z-axis). With reference to FIG. 12 b,it will be appreciated that FIGS. 13a-c each only depict four adjacentlines 44 within the product 40, which may include a greater number oflines 44 including the same configuration or pattern of line heights hrepeated along the X-axis. It will be appreciated that the configurationor pattern of lines 44 may not be the sole features of the product 40and that there may be different parts 42 having different features (e.g.whether in the form of lines 44 or other shapes and/or materials).

FIGS. 13a-c differ from each other in terms of the pattern of heights hof the lines 44. FIG. 13a illustrates a pattern of four lines 44 eachhaving an identical height h. FIG. 13b illustrates a pattern of fourlines 44 with a first and fourth line 44′ having a partially reducedheight h′ (in this example, a reduction in height of approximately 50%)and a second and fourth line 44 having the same height h as the lines 44in FIG. 13 a. FIG. 13c illustrates a pattern of four lines 44 with afirst line 44′ having a partially reduced height h′ (in this example, areduction in height of approximately 50% although other reductions maybe considered) and a second, third and fourth line 44 having the sameheight has the lines 44 in FIG. 13a -b. The pattern of four lines 44/44′may be repeated in the other lines of the part 42 and/or may be repeatedat other locations in the product 40. The lines 44′ that are reduced inheight h′ may define “first features” of the substrate W. The lines 44that are not reduced in height h may define “second features” of thesubstrate W.

In each of FIGS. 13a -13 c, a common pitch ‘p’ can be defined betweenthe lines 44, 44′. If no pitch walk is present then the common pitch pis identical for all of the lines 44, 44′. However, if pitch-walk ispresent, the common pitch p may change as a result of that error (e.g.such that the pitch between periodically spaced structures changes, forexample, by increasing S1 or S2 and decreasing the other of S1 or S2 bya corresponding amount). It will be appreciated that the depictedpatterns of four lines are merely examples and that anynumber/distribution of lines and/or other features may be repeated in apattern (e.g. two, three, five lines/features, and the like).

An example of a common pitch p includes 10 nm or less, 20 nm or less, 30nm or less, 50 nm or less, 100 nm or less, 150 nm or less, or 200 nm orless. Such a common pitch p may correspond to example feature sizes fora product on the substrate W. Thus, the present disclosure may describeapparatus, methods and/or systems for directly measuring at least oneparameter such as pitch walk at the feature level on a substrate W. Itwill be appreciated that for larger pitches, for example 300 nm or more,600 nm or more, the specular reflected radiation may include or beaccompanied by a signal comprising at least one diffractive order thatcan be measured using the optical apparatus.

As shown in FIGS. 13a -c, the lines 44′ represent first features and thelines 44 represent second features. It can be seen that the pitchbetween adjacent first feature 44′ is a multiple of the pitch p betweenadjacent second features 44, which is also defined as the common pitch.If pitch walk is present in the features 44, 44′ then the pitch p willvary, as explained above, because the values of S1 and S2 will change.However, the overall value of S1+S2 will remain essentially constant andis defined by the pre-patterned features 30 used to fabricate thefeatures 44, 44′. Therefore, for example in FIGS. 13b and 13 a, thepitch between the first features 44′ might remain essentially constant.This is seen more clearly when the spatially limited representation ofFIGS. 13b and 13c are extended to cover higher numbers of features 44,44′.

FIG. 14 depicts a schematic plan view of another example of a pattern oflines 44, for example, as may be included in the part 42 of FIG. 12 b.In this example, FIG. 14 is similar to FIG. 12 b, illustrating lines 44that extend along the Y-axis and are periodically distributed along theX-axis. In FIG. 14, two of the lines 44′ include cuts 46 within thelength (e.g. along the Y-axis) of the lines 44′ such that the lines 44′effectively include two separate lines separated by the cuts 46. Thecuts 46 in this example involve the complete removal of the lines 44′ inthe Z-axis. However, in another example, the cut 46 may involve onlypartial removal of the lines 44′ in the Z-axis (e.g. similar to FIGS.13b-c ) to reduce the height of the lines 44′. FIG. 14 illustrates apattern of four lines 44, 44′ with a second and third line 44′ eachhaving a cut 46 (in this example, a reduction in height of 100% andextending for only part of the length of the line 44′) and a first andfourth line 44 having the same height h as the lines 44 in FIGS. 13a -c.This pattern may be repeated in a similar manner to that described inrelation to FIGS. 13a -c. The lines 44′ that include cuts 46 may define“first features” of the substrate W. The lines 44 that are not reducedin height h may define “second features” of the substrate W.

Similar to the examples of FIGS. 13a -c, a common pitch ‘p’ can bedefined between the lines 44, 44′ in FIG. 14. It will be appreciatedthat the depicted pattern of four lines 44, 44′ and cuts 46 is merely anexample and that any number/distribution of lines, cuts and/or otherfeatures may be repeated in any pattern (e.g. two, three, fivelines/features, and the like). It will further be appreciated that aproduct 40 may include features having any CD and/or pattern. Forexample, the product 40 may include a pattern of features that include apartial reduction of height h′ and/or a cut 46 in at least one feature(e.g. a line 44, 44′) of the product 40.

FIGS. 15a and 15b depict a substrate W including a plurality of unitcells 48. Each unit cell 48 includes a pattern of features (e.g. lines44, 44′) that is repeated along the X-axis such that a combined pitch 50is defined between the first features 44′ and the second features 44across repeating unit cells 48. The unit cells 48 in FIGS. 15a-15binclude a pattern of four lines 44, 44′ that corresponds to the exampledepicted in FIG. 13 b, however, the unit cells 48 may be defined suchthat they include a different number of features 44, 44′. For example,using the exemplary arrangement of FIG. 13 c, the unit cell may bedefined to include 5 features 44, 44′ in total so as to provide asymmetrical unit cell. In the example of FIG. 15 a, the combined pitch50 is equal to ‘4p’ or four times the common pitch ‘p’ irrespective ofwhether pitch walk is present (i.e. the combined pitch 50 is equal forboth FIGS. 15a and 15b ). The combined pitch may be defined as thesmallest pitch that is an integer number of first features 44′ and alsoan integer number of second features 44. The combined pitch 50 in thepresent example does not effectively change if the unit cells 48 includelines 44, 44′ having an error indicative of pitch walk. The unit cells48 include a symmetric pattern/sequence of lines 44, 44′ about acenterline 51 of the unit cell 48. Either side of the centerline 51 is amirror pattern of lines 44, 44′.

It is noted that in the examples provided in the Figures and describedabove, the first features 44′ and the second features 44 are included ona single layer formed on the substrate W. In such examples, the firstfeatures 44′ may be formed by reducing the height of the second featuresas described. However, in other arrangements, the first features 44′ andthe second features 44 may be in separate layers formed on the substrateW. In those arrangements, the relative heights of the first and secondfeatures need not be significant in the measurement of pitch walk.Irrespective of whether the first and second features are formed on asingle layer or on separate layers, exemplary embodiments have pitchesbetween the first and second features such that the combined pitchremains essentially constant irrespective of the amount of pitch walkimparted to the features as part of the lithographic process.

As will be appreciated, when the first and second features are on asingle layer, the periodic nature of the second features 44 appears tobe interrupted by the first features 44′. However, for the purposes ofdescription, the pitch between the second features 44 is considered tobe common throughout because the first and second features are overlaid.

FIG. 16a depicts a system 60 for measuring a parameter (e.g. pitch-walk,CD, and the like) by illumination of the features (e.g. lines 44, 44′ orany other appropriate features) with measurement radiation from anoptical apparatus (see FIG. 3) and detecting a signal arising frominteraction between the measurement radiation and the features. Thesystem 60 includes a metrology apparatus 62 to measure the signal, forexample, to measure an optical response of the substrate W toillumination by the measurement radiation. The metrology apparatus 62may include the optical apparatus of FIG. 3 or any other appropriateapparatus, for example, an ellipsometer or the like for measuring atleast one selected from: angle, phase, amplitude, intensity,polarization information or the like of the signal.

The metrology apparatus 62 is configured to measure a parameter of adevice manufacturing process based on information acquired by theoptical apparatus or another appropriate apparatus. The opticalapparatus is configured to illuminate a plurality of features of thesubstrate W with measurement radiation and to detect a signal comprisingspecular reflected radiation arising from interaction between themeasurement radiation and the plurality of features by measuring thesignal using the optical apparatus, wherein the plurality of featuresare distributed in a periodic fashion defining a common pitch betweenadjacent features, and wherein one or more first features of theplurality of features has an at least partially reduced height comparedto one or more second features of the plurality of features. Themetrology apparatus 62 includes a processor 64. The processor 64 isconfigured to determine 66 an expected distribution of the specularreflected radiation, which may be in a pupil plane image. The expecteddistribution may define a “modeled optical response” 67 and arises frominteraction between the measurement radiation and the features (e.g.lines 44, 44′). The expected distribution is determined based on a model(e.g. an initial model 68 or an updated model 70). The expecteddistribution is calculated by, for example, solving Maxwell's equations(e.g. via a forward call) for the reflection of the signal from a modelof the features and its subsequent propagation through the opticalapparatus.

In the present example, the modeled optical response 67 comprises apupil plane representation 72 of the signal, the pupil planerepresentation 72 corresponding to an intensity distribution of thesignal at the pupil plane of the optical apparatus. However, it will beappreciated that the processor 64 could alternatively or additionally beconfigured to determine an image plane representation or otherrepresentation of the signal. The metrology apparatus 62 is configuredto measure 74 the distribution of the signal arising from interactionbetween the measurement radiation and the features (e.g. to provide a“measured optical response” 75). The processor 64 is configured tocompare 76 the image based on the signal (e.g. the measured opticalresponse 75) with the expected image (e.g. the modeled optical response)to determine an error therebetween. For example, the comparison 76 mayinclude determining a difference between corresponding pixel intensityvalues of the image based on the signal and the expected image (e.g. bysubtracting the difference). It will be appreciated that the comparison76 may use any appropriate method to determine the error.

The processor 64 may be configured to determine whether the modeledoptical response matches or substantially the measured optical response(e.g. by determining a fit 78). If the modeled and measured opticalresponses match or are sufficiently similar, the processor 64 mayindicate that ‘yes’ the fit 78 is a good or ‘best fit’ and, based on themodel of the substrate W features, reconstruct the feature or features.For example, as shown by FIG. 16 b, the processor 64 is configured toimplement the comparison 76. The processor 64 is configured to calculateif an error 80 between the expected and measured optical responses 67,75 is below a threshold 82, the processor 64 may be configured todetermine the parameter (e.g. pitch-walk, CD, or the like) to be aparameter 84 associated with the model 68 or 70. However, if the error80 is above the threshold 82, the processor 64 may be configured toupdate 86 the model 68, 70 in order to generate a new modeled opticalresponse 67 for comparison with the measured optical response 75 untilthe fit 78 is a good or ‘best fit’.

Upon determining the model 68, 70 that best fits with the measuredoptical response, it is possible to determine the parameter or aplurality of parameters of the substrate W (e.g. to reconstruct aprofile 88 such as CD or pitch-walk). If the best fit model 68, 70indicates that pitch walk (and/or another error) is present, then adecision may be made whether to recalibrate the lithographic apparatusLA, continue with the lithographic process, remove any layers of thesubstrate W containing the error and start again, or even remove thesubstrate W, or the like. Certain products may be able to tolerate acertain level of pitch walk (and/or another error) but other productsmay not be able to tolerate that certain level of pitch-walk (and/oranother error). Thus, the metrology apparatus 62 and associated methodsmay be capable of determining whether a lithographic manufacturingprocess is introducing an error into the products formed on thesubstrate W and take action at an appropriate time. Since error mayresult in the manufacture of lower quality or non-working products,determining error is a relevant part of the lithographic manufacturingprocess. The time taken to determine the error affects the efficiency ofthe manufacture of products. The metrology apparatus 62 and associatedmethods may be capable of determining the error faster than in priorexamples such that the lithographic manufacturing process may proceed ina time efficient and cost effective manner.

The system 60 may be implemented in any appropriate manner. An exampleof a system that may be used as part of the system 60 is described inPCT patent application publication no. WO 2015/082158 A1, the contentsof which is incorporated herein in its entirety by reference. PCT patentapplication publication no. WO 2015/082158 describes a reconstructionprocess that includes measuring structures formed on a substrate by alithographic process, determining a reconstruction model for generatingmodeled patterns, and computing and minimizing a multi-variable costfunction including model errors. Errors induced by nuisance parametersare modeled based on statistical description of the nuisance parameters'behavior, described by probability density functions. From thestatistical description, model errors are calculated and expressed interms of average model errors and weighing matrices. These are used tomodify the cost function so as to reduce the influence of the nuisanceparameters in the reconstruction, without increasing the complexity ofthe reconstruction model. The nuisance parameters may be parameters ofthe modeled structure, and/or parameters of an inspection apparatus usedin the reconstruction.

As depicted by FIG. 17, in an example reconstruction method 90 (whichmay be implemented using processor 64), such as CD reconstruction,pitch-walk reconstruction and/or reconstruction of at least one otherparameter, may include:

-   measuring a distribution of a signal as scattered by the features on    the substrate, e.g. by calculating a pupil image 92;-   defining a mathematical (e.g. a geometrical shape) model 94 (e.g.    such as model 68 or 70) of a feature such as a grating on a    substrate W;-   calculating 96 (e.g. integrating numerically) using Maxwell's    equations an expected distribution of a signal as scattered by the    model 94, e.g. to generate a calculated pupil image;-   comparing 98 the calculated pupil image with a measured pupil image    92 (e.g. as obtained by measurement using the metrology apparatus    62); and-   varying the model, e.g. by varying at least one parameter 100 of the    model influencing the expected pupil image (for example two or more    parameters 100 defining the geometrical shape of the features). The    above steps may be repeated until the estimated pupil image is    similar within a tolerance to the measured pupil image, at which    point the method 90 indicates that the model accurately describes    the feature or features of the substrate W to reconstruct 102 the    parameter of the device manufacturing process, e.g. pitch walk. It    will be appreciated that different parameters may affect the pupil    image in similar or different ways. For example, it may be possible    for at least two different parameters to produce the same pupil    image. In exemplary methods and apparatus disclosed herein, it may    be impossible to differentiate the effect of one parameter of the    features from another. In the example where pitch walk error is to    be determined, methods and apparatus disclosed herein allow the    effects of S1 and/or S2 on the pupil image to be determined    independently. It will also be appreciated that the method 90 may    implemented in various ways. For example, instead of measuring the    pupil image 92, an image plane (image) may be measured (or indeed    any image at any appropriate plane of the optical apparatus). The    mathematical model 94, calculating 96 step and/or the comparing 98    step may be appropriately modified or adapted to reflect the plane    of the optical apparatus that is being measured/calculated (e.g. due    to the different intensity distribution profiles at each plane of    the optical apparatus).

The present disclosure may provide a way to produce distinguishablepupil images (e.g. for each of the two parameters S1 and S2). Byproviding at least one feature of the substrate with a differentdimension (e.g. the partially reduced height of lines 44′ compared tolines 44 or the like), it may be possible to de-correlate, or calculatedistinguishable pupil images and identify that e.g. pitch-walk isoccurring in the features (in contrast to an error in the CD or anotherparameter). Conversely, the present disclosure may provide a way todetermine that at least one parameter of the features is responsible foran error or that at least one parameter is not responsible for the error(e.g. to assist in the identification of the error). The method 90 maybe at least partially implemented using at least part of the system 60or using any other appropriate system.

FIG. 18a corresponds to FIG. 13 a, with no reduction in height of any ofthe lines 44. That is, the features in FIG. 18a include only secondfeatures 44. FIGS. 18b and 18c respectively depict two “set-get” plotsshowing a simulation of the expected parameter (e.g. an expected valuefor S1 and S2) vs. a simulated parameter (e.g. a simulated value for S1and S2) for a substrate W including the features (e.g. a repeating unitcell) of FIG. 18 a. The x-axis of the plots indicate a “set” value andthe y-axis of the plots indicate a “get” value with both axes in nm. Inthis example, a real situation in a process is emulated by creating arandom perturbation (of ±2 nm on S1, S2 and CD) in the geometryparameters (including pitch walk). The simulated signal is thenprocessed e.g. as part of the system 60 or method 90 to assess whetherthere is enough signal to be able to determine the parameters S1 and S2,or pitch walk accurately. Using the structure illustrated by FIG. 18 a,the plots of FIGS. 18b and 18c indicate that it is not possible todetermine S1 and S2 as there is a poor R² correlation value between theexpected and simulated results (i.e. y=−0.1163x+11.088; R̂2=0.0089 forthe S1 get-set plot and y=−0.6649x+17.085; R̂2=0.3976 for the S2 get-setplot). The poor R² value indicates that a geometrical perturbation madeto the model does not provide enough signal in the pupil response withthe result that the S1 and S2 values cannot be inferred accurately.

FIG. 19 a, corresponds to FIG. 13 b, with a reduction in height of thefirst features (or lines) 44′. In this example, two “set-get” plots areillustrated by FIGS. 19b and 19c (respectively corresponding to theexpected/simulated parameters S1 and S2 for the structure illustrated byFIG. 19a ). The same procedure is applied to calculate the “set-get”plots as in FIGS. 18b and 18 c. However, in contrast, the set-get plotsindicate that it is possible to determine S1 and S2 accurately as thereis a very good R² correlation value between the expected and simulatedresults (i.e. y=0.9983x+0.0163; R²=0.9999 for the S1 get-set plot andy=0.9932x+0.0683; R²=0.9999 for the S2 get-set plot). Thus, thestructure provided by FIG. 19a (and other similar structures) may enablethe determination of S1 and S2, which might not otherwise be possible ifusing a structure such as shown by FIG. 18 a.

Any of the methods disclosed may be implemented using any appropriatelyconfigured metrology apparatus. The metrology apparatus 62 of thepresent disclosure may comprise an optical apparatus as discussed abovewith reference to FIG. 3 and/or any other optical apparatus, forexample, an ellipsometer or the like. A device manufacturing systemcomprising a device manufacturing apparatus and the metrology apparatusmay be provided. The device manufacturing system may comprise alithographic system comprising a lithographic apparatus LA and themetrology apparatus 62. The device manufacturing apparatus (e.g.including at least the lithographic apparatus LA) may perform a devicemanufacturing process (e.g. a lithographic manufacturing process) on asubstrate W. The metrology apparatus 62 may be configured to measure atleast one parameter of the device manufacturing process. The devicemanufacturing apparatus may use the parameter measured by the metrologyapparatus 62 in a subsequent device manufacturing process. Where theparameter represents an error in the device manufacturing process, thedevice manufacturing apparatus may use the parameter to reduce a size ofthe error or to indicate that an intervention needs to be made torectify or reduce the size of the error.

Further embodiments according to the invention are described in belownumbered clauses:

-   1. A substrate comprising a plurality of features for use in    measuring a parameter of a device manufacturing process by    illumination of the features with measurement radiation from an    optical apparatus and detecting a signal arising from interaction    between the measurement radiation and the features,    -   wherein the plurality of features comprise first features        distributed in a periodic fashion at a first pitch, and second        features distributed in a periodic fashion at a second pitch,        and    -   wherein the first pitch and second pitch are such that a        combined pitch of the first and second features is constant        irrespective of the presence of pitch walk in the plurality of        features.-   2. The substrate of clause 1, wherein the first features have been    fabricated using a spacer patterning method and wherein the combined    pitch is a multiple of a width of a mandrel used in the spacer    patterning method.-   3. The substrate of clause 1 or clause 2, wherein the first and    second features are spatially coincident in x and y dimensions of    the substrate.-   4. The substrate of any of clauses 1 to 3, wherein the first and    second features are fabricated on separate layers formed in or    patterned on the substrate.-   5. The substrate of any of clauses 1 to 3, wherein the first and    second features are fabricated on a single layer formed in or    patterned on the substrate.-   6. The substrate of clause 5, wherein one or more first features    comprises a second feature, the height of which has been at least    partially reduced.-   7. The substrate of clause 6, wherein the first features have a    reduced height compared to the second features over at least part of    a length and/or width of the first features in x and y dimensions of    the substrate, and wherein the height of the features is defined    along a corresponding z-axis, normal to the substrate.-   8. The substrate of clause 6 or clause 7, wherein a ratio of the    height of the first features to the second features is 0.9 or less;    0.8 or less; 0.7 or less; 0.6 or less; or 0.5 or less.-   9. The substrate of clause 8, wherein the ratio of the height of the    first features to the second features is 0.1 or more; 0.2 or more;    or 0.3 or more.-   10. The substrate of any of clauses 6 to 9, wherein the first    features have been formed by removal of at least part of one or more    of the second features to reduce their height, or by depositing at    least one layer on one or more of the second features to increase    their height.-   11. The substrate of any of clauses 6 to 10, wherein the first and    second structures form a plurality of repeating periodic unit cell    structures, each unit cell structure comprising at least one first    feature and at least one second feature.-   12. The substrate of clause 11, wherein the distribution of the at    least one first and second features in the unit cell is such that    the unit cell is symmetric.-   13. The substrate of clause 11 or clause 12, wherein the unit cell    has a width and/or length that is less than 150 nm; less than 100    nm; less than 80 nm; less than 60 nm; or less than 40 nm.-   14. The substrate of any of clauses 1 to 13, wherein the first and    second features are arranged such that a signal comprising specular    reflected radiation arising from interaction between the measurement    radiation and the first and second features comprises 0^(th)    diffraction order and is measureable using the optical apparatus to    determine the parameter of the device manufacturing process.-   15. The substrate of any of clauses 1 to 14, wherein the first and    second features are arranged such that a pitch walk error of the    first and/or second features produces distinguishable pupil images    in the pupil plane.-   16. The substrate of any of clauses 1 to 15, wherein the first and    second features comprise lines fabricated in or patterned on the    substrate by the device manufacturing process.-   17. The substrate of any of clauses 1 to 16, wherein the first pitch    is greater than the second pitch and is optionally a multiple of the    second pitch.-   18. The substrate of any of clauses 1 to 17, wherein the second    pitch is less than 100 nm; less than 80 nm; less than 60 nm; less    than 40 nm; less than 20 nm; or less than 10 nm.-   19. A substrate comprising a plurality of features for use in    measuring a parameter of a device manufacturing process by    illumination of the features with measurement radiation from an    optical apparatus and detecting a signal arising from interaction    between the measurement radiation and the plurality of features,    -   wherein the plurality of features are distributed in a periodic        fashion defining a common pitch of less than 100 nm between        adjacent features, and    -   wherein one or more first features of the plurality of features        has an at least partially reduced height compared to one or more        second features of the plurality of features.-   20. A metrology apparatus for measuring a parameter of a device    manufacturing process based on information acquired by an optical    apparatus, the optical apparatus configured to illuminate a    plurality of features of a substrate with measurement radiation and    to detect a signal comprising specular reflected radiation arising    from interaction between the measurement radiation and the plurality    of features by measuring the signal using the optical apparatus,    wherein the plurality of features comprise first features    distributed in a periodic fashion at a first pitch, and second    features distributed in a periodic fashion at a second pitch, and    wherein the first pitch and second pitch are such that a combined    pitch of the first and second features is constant irrespective of    the presence of pitch walk in the plurality of features, the    metrology apparatus comprising:    -   a processor configured to:    -   determine an expected distribution of the specular reflected        radiation, based on a model;    -   compare a measured distribution of the specular reflected        radiation with the determined expected distribution of the        specular reflected radiation to determine an error therebetween;        and    -   if the error is below a threshold, determine the parameter to be        a parameter associated with the model; or    -   if the error is above a threshold, update the model.-   21. The metrology apparatus of clause 20, wherein the measured    distribution of the specular reflected radiation is measured in the    pupil plane.-   22. The metrology apparatus of clause 20 or clause 21, wherein the    specular reflected radiation comprises 0^(th) order diffracted    radiation.-   23. A method of measuring a parameter of a device manufacturing    process based on information acquired by an optical apparatus, the    optical apparatus configured to illuminate a plurality of features    of a substrate with measurement radiation and to detect a signal    comprising specular reflected radiation arising from interaction    between the measurement radiation and the plurality of features by    measuring the signal using the optical apparatus, wherein the    plurality of features comprise first features distributed in a    periodic fashion at a first pitch, and second features distributed    in a periodic fashion at a second pitch, and wherein the first pitch    and second pitch are such that a combined pitch of the first and    second features is constant irrespective of the presence of pitch    walk in the plurality of features, the method comprising:    -   determining an expected distribution of the specular reflected        radiation, based on a model;    -   comparing a measured distribution of the specular reflected        radiation with the determined expected distribution of the        specular reflected radiation to determine an error therebetween;        and    -   if the error is below a threshold, determining the parameter to        be a parameter associated with the model; or    -   if the error is above a threshold, updating the model.-   24. A computer program comprising instructions which, when executed    on at least one processor, cause the at least one processor to    control an apparatus to carry out the method according to clause 23.-   25. A carrier containing the computer program of clause 24, wherein    the carrier is one of an electronic signal, optical signal, radio    signal, or non-transitory computer readable storage medium.

Although specific reference may have been made above to the use ofexamples of the present disclosure in the context of opticallithography, it will be appreciated that the present disclosure may beused in other applications, for example imprint lithography, and wherethe context allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), soft X-ray, as well as particle beams, such as ionbeams or electron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic, and electrostaticoptical components.

The foregoing description of the examples will so fully reveal thegeneral nature of the present disclosure that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific examples, without undueexperimentation, without departing from the scope of the presentdisclosure. Therefore, such adaptations and modifications are intendedto be within the meaning and range of equivalents of the disclosedexamples, based on the teaching and guidance presented herein. It is tobe understood that the phraseology or terminology herein is for thepurpose of description and not of limitation, such that the terminologyor phraseology of the present specification is to be interpreted by theskilled artisan in light of the teachings and guidance.

1. A substrate comprising a plurality of features for use in measuring aparameter of a device manufacturing process by illumination of thefeatures with measurement radiation from an optical apparatus anddetecting a signal arising from interaction between the measurementradiation and the features, wherein the plurality of features comprisefirst features distributed in a periodic fashion at a first pitch, andsecond features distributed in a periodic fashion at a second pitch, andwherein the first pitch and second pitch are such that a combined pitchof the first and second features is essentially constant irrespective ofthe presence of pitch walk in the plurality of features.
 2. Thesubstrate of claim 1, wherein the first features have been fabricatedusing a spacer patterning method and the combined pitch is a multiple ofa width of a mandrel used in the spacer patterning method.
 3. Thesubstrate of claim 1, wherein the first and second features arespatially coincident in x and y dimensions of the substrate.
 4. Thesubstrate of claim 1, wherein the first and second features arefabricated on separate layers formed in or patterned on the substrate.5. The substrate of claim 1, wherein the first and second features arefabricated on a single layer formed in, or patterned on, the substrate.6. The substrate of claim 5, wherein one or more first featurescomprises a second feature, the height of which has been at leastpartially reduced.
 7. The substrate of claim 6, wherein the firstfeatures have a reduced height compared to the second features over atleast part of a length and/or width of the first features in x and ydimensions of the substrate, the height of the features is defined alonga corresponding z-axis, normal to the substrate.
 8. The substrate ofclaim 6, wherein a ratio of the height of the first features to thesecond features is 0.9 or less.
 9. The substrate of claim 8, wherein theratio of the height of the first features to the second features is 0.1or more.
 10. The substrate of claim 6, wherein the first features havebeen formed by: removal of at least part of one or more of the secondfeatures to reduce their height, or by depositing at least one layer onone or more of the second features to increase their height.
 11. Thesubstrate of claim 6, wherein the first and second structures form aplurality of repeating periodic unit cell structures, each unit cellstructure comprising at least one first feature and at least one secondfeature.
 12. The substrate of claim 11, wherein the distribution of theat least one first and second features in each unit cell structure issuch that the unit cell structure is symmetric.
 13. The substrate ofclaim 11, wherein the unit cell structure has a width and/or length thatis less than 150 nm.
 14. The substrate of claim 1, wherein the first andsecond features are arranged such that a signal comprising specularreflected radiation arising from interaction between the measurementradiation and the first and second features comprises 0^(th) diffractionorder and is measureable using the optical apparatus to determine theparameter of the device manufacturing process.
 15. The substrate ofclaim 1, wherein the first and second features are arranged such that apitch walk error of the first and/or second features producesdistinguishable pupil images in the pupil plane.
 16. A metrologyapparatus for measuring a parameter of a device manufacturing processbased on information acquired by an optical apparatus, the opticalapparatus configured to illuminate a plurality of features of asubstrate with measurement radiation and to detect a signal comprisingspecular reflected radiation arising from interaction between themeasurement radiation and the plurality of features by measuring thesignal using the optical apparatus, wherein the plurality of featurescomprise first features distributed in a periodic fashion at a firstpitch, and second features distributed in a periodic fashion at a secondpitch, and wherein the first pitch and second pitch are such that acombined pitch of the first and second features is essentially constantirrespective of the presence of pitch walk in the plurality of features,the metrology apparatus comprising: a processor configured to: determinean expected distribution of the specular reflected radiation, based on amodel; compare a measured distribution of the specular reflectedradiation with the determined expected distribution of the specularreflected radiation to determine an error therebetween; and responsiveto the error being below a threshold, determine the parameter to be aparameter associated with the model, or responsive to the error beingabove a threshold, update the model.
 17. The metrology apparatus ofclaim 16, wherein the measured distribution of the specular reflectedradiation is measured in the pupil plane.
 18. The metrology apparatus ofclaim 16, wherein the specular reflected radiation comprises 0^(th)order diffracted radiation.
 19. A method of measuring a parameter of adevice manufacturing process based on information acquired by an opticalapparatus, the optical apparatus configured to illuminate a plurality offeatures of a substrate with measurement radiation and to detect asignal comprising specular reflected radiation arising from interactionbetween the measurement radiation and the plurality of features bymeasuring the signal using the optical apparatus, wherein the pluralityof features comprise first features distributed in a periodic fashion ata first pitch, and second features distributed in a periodic fashion ata second pitch, and wherein the first pitch and second pitch are suchthat a combined pitch of the first and second features is essentiallyconstant irrespective of the presence of pitch walk in the plurality offeatures, the method comprising: determining an expected distribution ofthe specular reflected radiation, based on a model; comparing a measureddistribution of the specular reflected radiation with the determinedexpected distribution of the specular reflected radiation to determinean error therebetween; and responsive to the error being below athreshold, determining the parameter to be a parameter associated withthe model, or responsive to the error being above a threshold, updatingthe model.
 20. A non-transitory computer program product comprisinginstructions, which when executed, are configured cause at least oneprocessor to control an apparatus to carry out the method according toclaim 19.